1. Field of the Invention
The present invention relates to ultrasonic cleaning apparatuses and methods, and more particularly to an ultrasonic cleaning apparatus and method for effecting the cleaning of contaminating substances from irregular and/or relatively large surfaces submerged in sea water or the like so that cleaning can be accomplished without removal of the surfaces from the sea water.
2. Description of the Prior Art
The accumulation of foreign substances such as biofouling on metal surfaces which are submerged in liquids is well known. Biofouling is ubiquitous and as used herein is meant to incorporate all types of marine bological and non-biological fouling, including phenomenon known in common terms as barnacles, slime, algae, etc. The most difficult type of biofouling to remove is that caused by plants and animals which secrete a calcareaus shell or exoskelton which by its nature becomes firmly cemented to the surface which is subject to the fouling.
When considering nautical apparatuses having components such as screws, rudders, boat hulls and the like, the effect of biofouling in terms of hydraulic drag is apparent. In order to remove or prevent biofouling several methods have been disclosed for such commonplace nautical instrumentalities.
One of the most widely used methods of preventing plant and animal associated fouling is the use of certain coatings which serve to halt growth for short periods of time or which, in some instances, serve only to slow growth. These paints or coatings work against biofouling by incorporating a soluble toxic compound that is released to poison biofouling organisms. The soluble toxic compounds are usually found in the salts of heavy metals such as copper, mercury, zinc, and arsenic. Unfortunately, these compounds have environmental complications and in some instance their use is severely restricted by Federal Regulations. In addition, these coatings have short lives and therefore the coating procedure must be repeated numerous times during the life of the maritime object to insure ongoing protection.
U.S. Pat. No. 4,058,075 issued to D. R. Piper, Sr. on Nov. 15, 1977, discloses another approach to the inhibition of marine life growth. Piper, Sr. suggests the use of several audio speakers attached to the inside of a boat hull or the like where the speakers are driven by 60 cycle AC so that they produce sound and therefore vibrations which inhibits or prevents marine growth on the exterior of the hull. While it is possible that this sytem will inhibit marine growth, it is quite unlikely that sufficient audio could be propagated through the hull to remove accumulations of biofouling. Therefore, this system must be in constant use in order to prove effective. Furthermore, the constant huming produced by the speakers would be a major annoyance to the occupants of the boat. In addition, this system is not readily adaptable for use over large areas or in configurations other than boat hulls, such as heat exchangers, since power levels required to be effective would be astronomical.
Another approach to the prevention of biofouling is disclosed in U.S. Pat. No. 3,309,167 issued to S. Gallar on Mar. 14, 1967. Gallar proposes the use of a heating element mounted on the interior of surfaces exposed to a marine environment. The heating of these surfaces acts as a deterrent for the growth of certain types of biofouling. However, the amount of power necessary to heat the entire surface of an object disposed in a marine environment is entirely impractical. Furthermore, if this system was to be used in conjunction with a heat exchanger submerged in a marine environment, the heat exchanger's function would be seriously impaired if not destroyed since heat transfer would not be properly accomplished through a heated surface, for whatever reason.
The question of preventing biofouling on a heat exchanger disposed in a marine environment was addressed in U.S. Pat. No. 4,062,189, issued to D. Mager et al on Dec. 13, 1977. Prior to Mager's proposal, the primary means of cleaning heat exchangers was the use of brushing or scraping implements or apparatuses which would mechanically attack biofouling and scrape the same from the surfaces of the heat exchanger. In an effort to advance the state of the art, Mager proposes to enhance the inherently inefficient method of mechanical scraping by reversing the cold and hot water flows over the heat exchanger periodically so that biofouling which is induced to grow is killed and then may be more easily scraped from the external surfaces of the heat exchanger.
Another proposal for cleaning biofouling from surfaces in contact with sea water is proposed in U.S. Pat. No. 3,068,829, issued to C. W. Nuissl, on Dec. 18, 1962. Nuissl teaches the use of an "Ultrasonic Energy Cleaner" which comprises a shell having disposed therein, what is called, "at least one high energy ultrasonic wave generator". The alleged "high energy ultrasonic wave generator" is actually an impeller driven by an electric motor mounted behind a plate having a plurality of holes disposed therethrough. The impeller draws water and forces it through the holes in pulses. As is well known in the ultrasonic arts, at best, the device of Nuissl can produce some type of streaming or water wave motion but because of the physical constraints of such an apparatus it cannot produce true ultrasonic cleaning or cavitation.
Ultrasonic cavitation cleaning in a marine environment is taught in U.S. patent application Ser. No. 776,578 filed on Mar. 11, 1977 by the U.S. Department of the Navy. This application has been made available prior to issue through the National Technical Information Service and teaches a thermal oceanographic sensor which is mounted on a substrate that comprises an ultrasonic transducer of the piezo-electric crystal type. When the ultrasonic transducer is energized, it effectively cleans itself and therefore cleans the thermal probe which is mounted thereon. No means are shown or suggested for propagating the ultrasound radiated from the transducer to clean surfaces other than its own.
In the area of general ultrasonic cleaning, U.S. Pat. No. 3,173,034 issued to C. W. Dickey et al on Mar. 9, 1965, discloses a low loss flexible ultrasonic transmission line for conducting ultrasonic energy to a remote location. U.S. Pat. No. 4,071,376 issued to L. M. McNear on Jan. 31, 1978 teaches an ultrasonic cleaner having a floating transducer which is configured in a doughnut-like shape for cleaning nuclear fuel casks. A plurality of transducers are disposed adjacent to the interior surfaces of the doughnut so that the nuclear fuel cask can be accommodated therethrough for cleaning.
The present invention addresses itself specifically to the cleaning of heat exchangers incorporated in ocean thermal energy conversion plants, however, the teachings thereof are specifically applicable to various other types of structures found in a marine atmosphere. The concept of an ocean thermal energy conversion plant is well known in the art. When the plant proposes to do is to extract thermal energy from the sun that is stored in tropical waters to generate electricity. These ocean thermal energy conversion systems are in effect giant Rankine engines which exploit the .DELTA.T between the surface of the ocean and depths several thousand feet therebeneath. It should be apparent, that in order to exploit this .DELTA.T, massive heat exchangers must be used, through or over which sea water must flow. The efficiency of the ocean thermal energy conversion plants is largely dependent upon the thermal transfer by the heat exchangers. Therefore, if biofouling accumulates on the heat exchanger and the heat transfer is reduced, the efficiency of the plant will be seriously impaired, possibly to the point where operation becomes implausible.
The ocean thermal energy conversion systems which rely upon sea water passing through the interior of the pipes of their heat exchangers have approached the problem of cleaning by passing abrasive pigs or sponges through the pipes under water pressure to scrape the interior surfaces thereof. Some quarters in the ocean thermal energy conversion plant field believe that the most practical and desirable plant will be one wherein the sea water flows over the exterior surfaces of the heat exchanger and a working fluid such as ammonia or the like is flowed through the interior surfaces of the exchanger. Unfortunately, this subjects the irregular and substantially inaccessible exterior surfaces of the heat exchanger to biofouling. To date, the only proposals which have been made for cleaning the exterior surfaces of these heat exchangers are the use of mechanical scraping or brushing means and the possible reversal of the heat exchange cycle so that areas are periodically changed from cold to hot. The reversal of the cycle is very impractical since the heat exchangers will most likely be in excess of 15,000 cubic feet each with a moderate size plant having an excess of 40 heat exchangers. The physical task of shifting the heat exchangers to reverse the flow would be almost unaccomplishable. Alternately, if the flow was to be reversed with the heat exchanger in position, massive plumbing would be required. Other systems which have been proposed include the use of water jets which are directed against the exterior surfaces of the heat exchanger to prevent bifouling from accumulating thereon. However, it is known that these systems will be entirely ineffective in removing accumulated deposits and the energy as well as the pumping system required to keep them constantly running to permit accumulations will be prohibitive. In addition, if there are some malfunctions and the system cannot be used continuously, the biofouling which will accumulate during the down time will be virtually impossible to remove. Conventional techniques such as coatings are undesirable for use with heat exchangers because they reduce heat transfer in some instances and also have environmental complications. Environmental complications are also the major concern with chemical systems wherein a toxic substance is periodically disbursed to preclude accumulation of biofouling.
Considering all of the above complications and problems, the primary consensus in the ocean thermal energy conversion field has been that mechanical brushing or scrubbing seems the most likely candidate for cleaning of biofouling from the heat exchangers of the ocean thermal energy conversion plants, however, inefficient and cumbersome these mechanical systems might be.
The present invention overcomes the problems associated with the prior art and fulfills the need for efficiently and effectively cleaning biofouling from the external surfaces of the heat exchangers of ocean thermal energy conversion plants by providing an ultrasonic cleaning apparatus which can be used in combination with a heat exchanger to accomplish complete and effective removal of biofouling. In addition, the apparatus of the present invention, when employed with the method thereof, is suited for applications other than the cleaning of heat exchangers.